Field of the Invention
The present invention relates to a sheet feeding device for feeding stacked sheets, and an image forming apparatus including the sheet feeding device.
Description of the Related Art
An image forming apparatus such as a copying machine, a printer, and a facsimile includes a sheet feeding device in which a plurality of pieces of sheet-shaped recording paper (hereinafter, referred to as sheets) is stored. The image forming apparatus takes out and feeds the sheets one by one from the sheet feeding device to an image forming unit, and performs image formation on the sheets. The sheet feeding device includes a sheet feeding cassette (sheet feeding tray) for stacking a sheet bundle thereon. In the sheet feeding cassette, an elevating tray on which the sheet bundle is stacked is lifted up toward a sheet feeding roller by a spring or gear configuration. The sheet feeding roller makes press-contact with the top surface of the lifted sheet bundle and rotates to feed a sheet out of the sheet feeding cassette. If two sheets are simultaneously fed out by the sheet feeding roller, a separation roller separates the sheets and the upper sheet is conveyed to a conveyance path leading to the image forming unit.
In the sheet feeding cassette, the elevating tray generally descends when a user opens (pulls out) the sheet feeding cassette to replenish sheets. The reason is that sheets are not able to be replenished if the elevating tray or the sheets on the elevating tray are in press-contact with the sheet feeding roller. After the user replenishes sheets and closes (pushes in) the sheet feeding cassette, a lift-up operation is performed by which the elevating tray ascends toward the sheet feeding roller.
For example, a sheet feeding device discussed in Japanese Patent Application Laid-Open No. 2007-238312 includes a position sensor capable of detecting the position of sheets and an elevation tray inside. For example, when the sheet feeding device performs a lift-up operation of the sheets and the elevating tray, the elevating tray ascends at a first speed until the position sensor detects the ascent of the elevating tray to a predetermined position. After the position sensor detects that the elevating tray has reached the predetermined position, the elevating tray ascends at a second speed lower than the first speed. This reduces the time needed for the lift-up operation and improves sheet position accuracy at the completion of the lift-up operation.
Provision of the dedicated sensor for detecting the timing to switch the lift-up speed leads to increased costs. The sensor for detecting the timing to switch the speed may be omitted for the sake of cost reduction. In such a case, however, the time to complete the lift-up operation increases and usability decreases if the lift-up speed is low. If the lift-up speed is uniformly increased, variations in sheet position at the completion of the lift-up operation may increase, thereby reducing the stability of sheet feeding operations. More specifically, a sheet feeding pressure between a sheet and the sheet feeding roller that are in the press-contact state is extremely important to stably separate and convey sheets one by one from a sheet bundle. Variations in the sheet position make sheet feeding operations unstable.